This invention relates to the use of a piezoelectric (PZ) transducer as a method of measuring the energy of a pulsed laser and an apparatus that can measure this energy.
Conventional power and energy meters, for high power pulsed lasers include various thermal and electro-optic devices designed to convert all or part of the pulse energy into thermal or electrical energy. A typical high power laser power meter will consist of a thermopile (a device utilizing thermocouples for converting thermal energy into electrical energy) and a device for monitoring the electrical output of the thermopile where it is absorbed and causes a temperature increase in the material of the thermopile. As the temperature rises, the array of thermocouples generate an electric potential which is sensed by some sort of voltage metering device.
The length of time for such systems to come to equilibrium after the introduction of the laser beam is typically quite long (perhaps minutes). By introducing a series of pulses from a pulsed laser, the energy of the pulses may be integrated to produce a total energy for the entire sequence of pulses. The individual pulse is inferred by dividing the total energy measured by the number of pulses. However, such measurement assumes equal pulse energy, which may or may not be valid. Finally, such measurement devices typically encounter relatively severe problems with "drift" in the laser output caused by environmental temperature variations, instabilities in the laser control electronics or other effects, all of which are exacerbated by the relatively slow response time of the devices.
More sophisticated devices such as photodiodes, which measure laser power directly by impinging the laser beam on the measuring device, are prone to damage, especially when impacted with a beam from a high power laser.
A particularly vexing problem arises in the measurement of the energy of a rapidly pulsed laser, especially when large energies or power levels are involved. In such cases, one of two measurement approaches has conventionally been taken. Firstly, by monitoring the relative brightness of the successive pulses, while measuring the total energy in the pulse train with a thermal power meter, one can distribute the total measured energy among the various pulses based on the relative brightness of the pulses. Secondly, one can measure the brightness of the individual pulses using a conventional optical meter. The brightness is related to the pulse energy and is a measure of such energy. In using this method, all or part of the photons contained in the laser pulse fall on a detector material such as silicon or germanium, and their energy is converted into an electrical signal proportional to the number and energy of the photons. It should be noted that different detector materials must be used for different portions of the optical spectrum. Detector materials for visible radiation include silicon and germanium photodiodes, while for the infrared spectrum, materials such as HgCdTe, InSb, InGaAs, PbS and InAs, among others, have been used as photon detectors. Another class of photon detector are photomultiplier tubes (PMT), however, these have extreme sensitivity which generally precludes their use in high power measurements.
Problems exist with each of these approaches, especially with higher pulse energies. In particular, the direct measurement of the brightness of laser pulses becomes very difficult due to the limited dynamic range of typical photon sensors. Additionally, small portions of the pulse must be separated (the beam must be "split") for measurement in a way that allows one to relate this partial energy back to the true total energy of the original pulse. Actual damage to the optical detector is a distinct possibility when energies exceed damage thresholds, as may happen when unknown pulses or pulses having large energy variations must be measured.
U.S. Pat. No. 4,820,047, issued Apr. 11, 1989, discloses a laser heterodyne apparatus for measuring optical power. A detection system is provided which has a very wide linear dynamic range (for a typical laser, fifteen orders of magnitude), as well as a high sensitivity (shot. noise limited), high angular resolution (diffraction limited), and which is also polarization resolving. The apparatus is used for measuring optical power, and includes a system of producing two optical beams such that at some point and thereafter along an optical path of the beams, a frequency difference exists between the two beams. One of the two beams has a known or constant optical power, and the other of the two beams is the beam whose power is to be measured. A combining element for coherently combining the two optical beams is included. An optical detection system receives the combined optical beam, and in response provides an electrical signal modulated at the difference between the frequencies of the optical fields of the two optical beams. Additionally, U.S. Pat. No. 4,474,468, issued Oct. 2, 1984, discloses a method and apparatus for measuring the power of a laser beam. A thin wire is arranged across the laser beam, and the change in resistance of the wire is measured by a resistance meter while moving the wire by a driving mechanism, thereby measuring the power of the laser beam or the position of the wire in the laser beam. The apparatus can be used for monitoring the power of the laser beam or the position of the beam.